Electrical apparatus with corona suppression means

ABSTRACT

In electrical apparatus in which high voltages may occur between two closely spaced conductive members, at least one of which is insulated by a coating of insulation, and in which the electric field gradient parallel to the surface of this insulated conductor in some area is sufficient to cause surface corona, an additional member of electrical insulation is introduced to this area to form a new insulation surface with a lower field gradient, thus suppressing surface corona. To suppress end corona between the end portion of a stator coil and the stator core of a dynamoelectric machine, a collar encircling the coil at the face of the core is provided in accordance with this invention. The invention facilitates the production of high voltage machines in which the coils are insulated by post impregnation (i.e., after winding on the core).

United States Patent Fort 54] ELECTRICAL APPARATUS WITH CORONASUPPRESSION MEANS Emil M. Fort, Murrysville, Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

[22] Filed: Feb. 2, 1971 [2l] Appl.No.: 111,811

[72] Inventor:

[73] Assignee:

52 us. Cl ..1.....310/196, 310 45 France ..3l0/l96 51 July 25, 1972Primary Examiner-D. X. Sliney Attorney-A. T. Stratton [5 7] ABSTRACT Inelectrical apparatus in which high voltages may occur between twoclosely spaced conductive members, at least one of which is insulated bya coating of insulation, and in which the electric field gradientparallel to the surface of this insulated conductor in some area issufficient to cause surface corona, an additional member of electricalinsulation is introduced to this area to form a new insulation surfacewith a lower field gradient, thus suppressing surface corona. Tosuppress end corona between the end portion of a stator coil and thestator core of a dynamoelectric machine, a collar encircling the coil atthe face of the core is provided in accordance with this invention. Theinvention facilitates the production of high voltage machines in whichthe coils are insulated by post impregnation (i.e., after winding on thecore).

6 Claims, 3 Drawing Figures PATENTEDJULZS m2 sum 1 or 2 INVENTOR Emil M.Fort ATTORNEY WITNESSES .iymagb Q PATENTEDJULZS m2 3.679.925 SHEET 2 0r2 TAN DELTA (70) o 5 IO VOLTAGE (KILOVOLTS) ELECTRICAL APPARATUS WITHCORONA SUPPRESSION MEANS BACKGROUND OF THE INVENTION 1. Field of theInvention The invention relates to electrical apparatus with highvoltages between closely adjacent parts and particularly to high voltagecoils for dynamoelectric machines constructed to avoid occurrence ofcorona during use thereof.

2. Prior Art Corona is a type of gaseous electrical discharge occurringwhen the electrostatic potential gradient along the surface ofelectrical insulation in air or another gaseous medium exceeds a certainvalue. Corona results in lost power in the apparatus and can deteriorateinsulation to the point of failure. High voltage dynamoelectricmachines, such as those in which the windings on the stator are atpotentials of the order of 10,000 volts or more, in relation to thegrounded stator core, are recognized to be quite susceptible to coronaand extra care in the design of the machine is required for itsavoidance.

With respect to the stator coils in high voltage machines, surfacecorona can occur in three particular locations. The radial electricfield gradient between the coil conductor and the stator core can causeslot corona in any air gap occurring between the coil surface and thesurface of the core slot. Corona can also occur at the edges of ventgaps provided in the stator core, such corona being due to local axialfield gradients. A third type of corona is end corona occurring wherethe coil leaves the core slot and is caused by the considerable axialelectric field gradient at the coil surfaces in this location.

For high voltage stators whose coils are wound from fully insulatedconductors, methods to suppress corona are now well known. The slotcorona and the vent gap corona can be suppressed with a relatively lowresistance, semiconductor coating or finishing tape on the coils. Theend corona can be suppressed with either a high resistance semiconductorcoating on the end portions of the coil or with a capacitor gradingsystem. Manni US. Pat. No. 2,939,976, June 7, 1960, may be referred tofor some of the background concerning corona suppression in machines inwhich the coils are wound from insulated conductors.

There is increasing interest in providing the insulation in high voltagemachines by a process sometimes referred to as post-impregnation. Bypost-impregnation is meant the application of an insulating medium, byvacuum impregnation, to conductors that have previously been wound intocoils on the stator core. In post-impregnation processes the same typesof insulation may be used as in the case of conductors impregnatedbefore being placed on the stator core. For example, the individualconductive strands may be insulated by means such as glass servings orenamel. A conductor, usually comprising a plurality of such strands, iswrapped over its length with insulating tape, typically including one ormore layers of mica tape followed by one or more layers of additionaltape, such as a glass tape, to mechanically secure the mica tape. Normalvacuum impregnation is performed on a length of such a conductor (e.g.by immersing the conductor in a suitable resin, evacuating theenclosure, and curing) after which it is placed in the stator core slotto form a coil. Post-impregnation is similarly performed except that theconductor, wrapped with tape as before, is wound on the stator corebefore the impregnation process. This provides uniform insulation overthe entire wound core in one operation, prevents air gaps within theslots and avoids the need to make connections between one-impregnatedcoil segments.

Unfortunately, prior techniques for corona suppression are not readilyapplicable to stators that are manufactured by post-impregnation. Slotcorona is not a problem in such machines as the impregnation should notleave any air gap between the coils and the slot surface. Also, asemiconducting finishing tape with a suitable conductivity could beapplied after impregnation to suppress vent gap corona. The impregnationof a previously wound stator, however, makes difficult the use of acontinuous surface coating, either before or after impregnation, as iscommonly used on fully insulated (pre-impregnated) coils. Thus, asolution for the suppression of end corona in such machines has notpreviously been available. A sufficiently high resistance, permeabletape that could be applied before impregnation is difficult to obtainand the use of capacitive grading is not very practical for voltagesbelow about 18,000 volts which can still be sufficiently high to causecorona.

SUMMARY OF THE INVENTION The present invention is primarily directed tomeans for the suppression of end corona in those instances, such as inthe case of post-impregnated stator coils, where prior means have beenineffective. In accordance with this invention, a collar of electricallyinsulating material is disposed to completely encir cle the conductor ina manner that is in contact with both the conductor and any previouslyapplied insulating coating thereon and the adjacent conductive bodiessuch as the stator core so as to decrease the maximum electric fieldgradient on the surface of the insulation separating those twoconductive members.

The invention can be readily applied in machines in which the coils areinsulated by post-impregnation. A way in which this can be done is toapply an encircling collar around the wound coil in essentially thedesired position with a subsequent impregnation process causing anyvoids to be filled between the collar and the adjacent conductors andalso to cause the collar to adhere firmly thereto. Alternatively, thecorona suppression collar can be affixed after impregnation of the coilwith the desired function achieved so long as care is taken to avoid anyair gap between the collar and adjacent surfaces.

THE DRAWING FIG. I is a partial view, in perspective, of electricalapparatus in accordance with the present invention;

FIG. 2 is a cross-sectional view of a portion of the structure shown inFIG. I; and

FIG. 3 is a graph illustrating curves to show the effectiveness of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2,there is shown a portion of electrical apparatus, such as part of thestator of a dynamoelectric machine. The stator includes a magnetic core10 that comprises a plurality of stacked laminations 11. In the portionof the structure shown, the magnetic core 10 is provided with a slot 12within which are positioned upper and lower coils l4 and 15. The ends ofeach of the coils I4 and 15 are here sectioned to reveal the internalstructure. A coating l6 of insulating material 16 surrounds theindividual coils 14 and 15 both within and outside of the slot 12. Thecoils respectively comprise portions l4a and 15a within the slot 12 aswell as end portions 14b and 15b extending outward from an end face 17of the magnetic core. A conventional slot wedge 19 of insulatingmaterial may be provided to secure the coils within the core. Aconventional filler strip 21 of insulating material (eg mica) may, also,be disposed between the two coils l4 and 15.

Reference numeral 16 identifies the insulation system provided betweenthe coils I4 and 15 and the core 10 and also that over the end turns 14band 15b. While sometimes referred to herein as a coating or layer 16, itis to be understood that it may consist of a plurality of elements. Forexample, it may have three successive portions: first, the insulation onthe inamenable to corona suppression means as have been previouslyemployed. Thus, completion of the insulation coating or system 16 bypost-impregnation leaves a corner" surrounding the coils l4 and 15 atwhich end corona is likely to occur. This corner or space would normallybe occupied by the air or other gaseous medium in which the machine isdisposed.

As an incidental result of post-impregnation, the insulation applied inthe impregnation process will coat all exposed parts of the stator coreincluding end face 17. Such a coating on end face 17 has no appreciableeffect on the likelihood of corona and is ignored in the drawing.

In accordance with this invention, the space adjacent the face 17 andcoils 14 and I5 is filled by a collar 18 of insulation material that isdisposed around each of the coils l4 and 15 in contact with theinsulation coating 16 of the end turns 14b and 15b, and against the face17 of the core 10. The electric field gradient on the surface of thiscollar will now be considerably lower than that on the initialinsulation surface without the collar.

In dynamoelectric machine operation, the coils l4 and 15 may carryvoltages up to substantially in excess of l0,000 volts while the statorcore 11 is at ground potential. The inception voltage, which is thatvoltage at which end corona is initiated, is approximately proportionalto Us, where t is the thickness and e is the dielectric constant of theinsulation. Here that effective thickness of insulation is substantiallyincreased.

The outer dimensions of collar 18 may be chosen in accordance with theneed. In general, a collar extending a greater distance along face 17from 12 and a greater distance along the coils l4 and 15 away from theface will be more effective. Thicknesses (in the directions justindicated) about five times that of the impregnated insulation coating16 (which is typically about 100 mils to 200 mils, thick) would besuitable in many applications.

In accordance with this invention, the insulation materials for theimpregnated coils and for the collar may be selected from various knownmaterials. The impregnated coil insulation can be applied from a fluidin an evacuated chamber and comprise various resinous materials such aspolyester, epoxy or silicone resins. The insulating collar 18 may be ofa preformed solid member of similar composition or of a differentcomposition compared to that used in the post-impregnation process. Itis suitable to employ a collar of absorbent material, for instance,polyester felt, that is fixed in position before impregnation and wouldbe saturated with the resin employed in the impregnation process.

The invention has been demonstrated by constructing a model of a statorcore in which two test bars were disposed in a slot and impregnated withan epoxy resin. The total amount of corona occurring both internally andat the end where the bars extended from the core was evaluated by theknown method of using a Schering bridge and measuring the tangent of thephase angle 8 occurring when the bridge is in balance for various valuesof applied voltage between the bars and the core. The results are shownin FIG. 3. Curve A illustrates the results occurring over a range offrom 1000 to 10,000 volts for such a configuration without a coronasuppression collar in accordance with this invention. Curve Billustrates the results with the same structure after reimpregnation ofa collar of a polyester felt material around the bars. These curves showclear improvement in the structure in accordance with this invention.

While particularly advantageous in machines made using apost-impregnation process, the invention is not so limited, i.e.,insulating coating 16 may be applied by other means such as fullinsulation applied to individual conductors before winding. In any eventthe collar 18 will have the effect of reducing the voltage gradientbetween the core 11 and coils l4 and 15.

While the invention has been primarily discussed to the problem ofcorona suppression in dynamoelectric machines,.

or the stator coils of such machines, it is to be understood that it canbe applied in other electrical apparatus such as for exam le highvoltage bushings.

hat IS claimed is:

insulation collar around said coil and in contact with the insu-,

lation on said coil and that on said end face, said collar completelyencircling said end portion of said coil and occupying a volume throughwhich the shortest paths between said end face and said end portion ofsaid coil occur.

2. The subject matter of claim 1 wherein: said collar is of an absorbentinsulating matrix impregnated with a resinous insulating material thatalso comprises a component of the insulation in which said conductiveturn is embedded.

3. The subject matter of claim 1 wherein: said magnetic core of saidstator and said coil are operated with a voltage difference therebetweenof at least 10,000 volts.

4. A method of making electrical apparatus capable of operation at highvoltages with low probability of corona, comprising the steps of:assembling electrically conductive members of the apparatus to beoperated at substantially different voltages; placing a continuousmember of an insulating material in corners between said conductivemembers that otherwise would be occupied by the gaseous ambient in whichthe apparatus operates; impregnating the assembled conductive memberswith an insulation medium to provide solid insulation between saidconductors and to securely bond said member of insulating material tosaid conductive members without a gap therebetween. 1

5. The subject matter of claim 4 wherein: said member of insulatingmaterial is a preformed solid member of insulating material.

6. The subject matter of claim 4 wherein: said member is of an absorbentinsulating matrix.

1. A high voltage dynamoelectric machine comprising: a stator includinga magnetic core with at least one slot extending therethrough, a coil ofan electrical conductor wound around said core with a portion disposedwithin said slot and an end portion extending into a gaseous mediumbeyond an end face of said core, said coil comprising at least oneconductive turn embedded in a mass of insulation by impregnation aftersaid turns are wound on said core, a corona suppressing insulationcollar around said coil and in contact with the insulation on said coiland that on said end face, said collar completely encircling said endportion of said coil and occupying a volume through which the shortestpaths between said end face and said end portion of said coil occur. 2.The subject matter of claim 1 wherein: said collar is of an absorbentinsulating matrix impregnated with a resinous insulating material thatalso comprises a component of the insulation in which said conductiveturn is embedded.
 3. The subject matter of claim 1 wherein: saidmagnetic core of said stator and said coil are operated with a voltagedifference therebetween of at least 10,000 volts.
 4. A method of makingelectrical apparatus capable of operation at high voltages with lowprobability of corona, comprising the steps of: assembling electricallyconductive members of the apparatus to be operated at substantiallydifferent voltages; placing a continuous member of an insulatingmaterial in corners between said conductive members that otherwise wouldbe occupied by the gaseous ambient in which the apparatus operates;impregnating the assembled conductive members with an insulation mediumto provide solid insulation between said conductors and to securely bondsaid member of insulating material to said conductive members without agap therebetween.
 5. The subject matter of claim 4 wherein: said memberof insulating material is a preformed solid member of insulatingmaterial.
 6. The subject matter of claim 4 wherein: said member is of anabsorbent insulating matrix.